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Abstract:

A rolling bearing is provided. The rolling bearing includes an inner
ring, an outer ring, a plurality of rolling elements disposed in a space
between the inner ring and the outer ring, and a grease composition
enclosed in the space. The grease composition including (A) a lithium
soap thickener, (B) a base oil which is an ester synthetic oil obtained
by an esterification reaction of trimethylolpropane with a fatty acid;
and (C) (c-1) a 1-naphthylamine antioxidant and (c-2) a diphenylamine
antioxidant, a total amount of (c-1) and (c-2) ranging from 1.0 to 10
mass % based on an entire amount of the grease composition, and a
blending ratio of (c-1) and (c-2) ranging from 30:70 to 70:30 in terms of
mass ratio.

Claims:

1. A rolling bearing comprising an inner ring, an outer ring, a plurality
of rolling elements disposed in a space between the inner ring and the
outer ring, and a grease composition enclosed in the space, wherein the
grease composition comprises: (A) a lithium soap thickener; (B) a base
oil which is an ester synthetic oil obtained by an esterification
reaction of trimethylolpropane with a fatty acid; and (C) (c-1) a
1-naphthylamine antioxidant and (c-2) a diphenylamine antioxidant, a
total amount of (c-1) and (c-2) raging from 1.0 to 10 mass % based on an
entire amount of the grease composition, and a blending ratio of (c-1)
and (c-2) ranging from 30:70 to 70:30 in terms of mass ratio.

2. The rolling bearing according to claim 1. wherein (c-1) the
1-naphthylamine antioxidant is a compound represented by the following
formula (I): ##STR00005## wherein in the formula (I), each of R1 and R2
are mutually independent and are a hydrogen atom, or a linear or branched
alkyl group having a carbon number of 1 to 12.

3. The rolling bearing according to claim 1, wherein (c-2) the
diphenylamine antioxidant is a compound represented by the following
formula (II): ##STR00006## wherein in the formula (II), each of R3 and
R4 are mutually independent and are a hydrogen atom, or a linear or
branched alkyl group having a carbon number of 1 to 12.

4. The rolling bearing according to claim 1, wherein (B) the ester
synthetic oil is an ester oil obtained by an esterification reaction of
trimethylolpropane with a mixture of linear and branched fatty acids each
having a carbon number of 6 to 20.

Description:

[0002] The present invention relates to a rolling bearing, in particular,
a rolling hearing suitable for a small motor.

[0003] 2. Description of the Related Art

[0004] Power saving and high durability are required for a small motor
such as a brushless motor and a fan motor used in home electrical devices
or information devices, and low noise, long acoustic life and the like
are required for a rolling bearing used in such a small motor.

[0005] As a grease composition for achieving low noise and long acoustic
life in a rolling bearing, there is known a grease composition prepared
by incorporating a cleaning dispersant into grease with an attempt to
prevent abrasion powder due to running-in wear from adhering to a rolling
contact surface and thereby achieve low noise and long acoustic life (for
example, refer to JP 4117445 B).

[0006] There is also known a grease composition in which a thickener
composed of a urea compound having a specific composition and a specific
organic sulfonate are combined, thereby making it possible to improve the
initial acoustic characteristics and also prevent the acoustic
characteristics from reducing with aging or reducing due to a temperature
rise (for example, refer to JP 2008-143979 A).

[0007] In general, lithium soap-based grease is used for applications
requiring acoustic characteristics such as low noise, and urea-based
grease is used for applications requiring heat resistance. However,
neither the lithium-based grease nor the urea-based grease can satisfy
both heat resistance and long acoustic life, and therefore, a bearing
having enclosed therein such grease has a problem that during use at a
high temperature, the time period maintaining the initial acoustic
characteristics would be shortened.

[0008] Accordingly, there is a strong demand for a rolling bearing capable
of achieving a long acoustic life even when used in a high-temperature
environment that is likely to be produced in a small motor or the like.

SUMMARY OF THE INVENTION

[0009] The present invention has been made in view of the above
circumstances, and an object of the present invention is to provide a
rolling bearing having a heat-stable long acoustic life.

[0010] According to an illustrative embodiment of the present invention,
there is provided a rolling bearing comprising ah inner ring, an outer
ring, a plurality of rolling elements disposed in a space between the
inner ring and the outer ring, and a grease composition enclosed in the
space,

[0011] wherein the grease composition comprises:

[0012] (A) a lithium soap thickener;

[0013] (B) a base oil that is an ester synthetic oil obtained by an
esterification reaction of trimethylolpropane with a fatty acid; and

[0014] (C) (c-1) a 1-naphthylamine antioxidant and (c-2) a diphenylamine
antioxidant, a total amount of (c-1) and (c-2) raging from 1.0 to 10 mass
% based on an entire amount of the grease composition, and a blending
ratio of (c-1) and (c-2) ranging from 30:70 to 70:30 in terms of mass
ratio.

[0015] In the above grease composition, (c-1) the 1-naphthylamine
antioxidant may be a compound represented by the following formula (I):

##STR00001##

[0016] wherein in the formula (I), each of R1 and R2 are mutually
independent and may be a hydrogen atom, or a linear or branched alkyl
group having a carbon number of 1 to 12.

[0017] In the above grease composition, (c-2) the diphenylamine
antioxidant may be a compound represented by the following formula (II):

##STR00002##

[0018] wherein in the formula (II) each of R3 and R4 are mutually
independent and may be a hydrogen atom, or a linear or branched alkyl
group having a carbon number of 1 to 12.

[0019] In the above grease composition, (B) the ester synthetic oil may be
an ester oil obtained by an esterification reaction of trimethylolpropane
with a mixture of linear and branched fatty acids each having a carbon
number of 6 to 20.

[0020] In the above grease composition, (A) the lithium soap thickener may
be lithium 12-hydroxystearate.

[0021] According to the above configuration, a rolling bearing having a
heat-stable long acoustic life is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a view showing a schematic configuration of a ball
bearing according to an illustrative embodiment of the present invention.

DETAILED DESCRIPTION

[0023] The configuration of a ball bearing according to an illustrative
embodiment of the present invention is described in detail below by
referring to the attached drawing.

[0024] FIG. 1 is a cross-sectional view in a diameter direction of a ball
bearing 1 according to an illustrative embodiment of the present
invention. The ball bearing 1 has the same basic structure as a
relate-art ball bearing and includes an annular inner ring 2 and an
annular outer ring 4. The outer ring 4 has a larger diameter than the
inner ring 2 and is disposed concentrically with the inner ring 2 to form
an annular space 6 between the outer ring 4 and the inner ring 2.

[0025] In the annular space 6, an annular retainer 10 for holding a ball 8
is disposed. The retainer 10 has a plurality of concaves disposed at
fixed intervals in the circumferential direction, and a ball 8 is housed
in each concave. In each concave, the ball 8 is disposed to allow itself
to contact the inner ring 2 at one side and the outer ring 4 at another
side.

[0026] Furthermore, at both ends of the annular space 6 in the axis line
direction, annular seal members 12, 12 are provided respectively, whereby
the annular space is sealed in the axis line direction.

[0027] As a result, the annular space 6 becomes a closed space defined by
the inner ring 2 in the radially inside direction, the outer ring 4 in
the radially outside direction, and the annular seal members 12, 12 at
both ends in the axis line direction. Each of the annular seal members
12, 12 is an annular member made of a steel sheet, and its outer edge
part is attached to the outer ring 4.

[0028] In the ball bearing 1 of this illustrative embodiment, a grease
composition 14 according to the later-described formulation is enclosed
in the annular space 6. The amount of the grease composition enclosed is
preferably in the order of 25 to 35% of the volume of the annular space
6.

[0029] The grease composition enclosed in the annular space 6 of the ball
bearing 1 according to this illustrative embodiment is described below.

[0030] The grease composition used for the ball bearing 1 of this
illustrative embodiment includes (A) a lithium soap thickener, (B) a base
oil, and (C) an antioxidant, which are described below.

[0031] <Grease Composition>

[0032] [(A) Lithium Soap Thickener]

[0033] The lithium soap thickener used in the grease composition for the
ball bearing 1 of this illustrative embodiment has fewer drawbacks and is
not expensive and therefore, this is a practical thickener. The thickener
is preferably lithium laurate, lithium stearate or lithium
12-hydroxystearate, more preferably lithium laurate or lithium
12-hydroxystearate, still more preferably lithium 12-hydroxystearate. The
rolling bearing having enclosed therein a grease composition including
the lithium soap thickener is excellent in the low noise property.

[0034] The consistency of the grease composition used for the ball bearing
1 of this illustrative embodiment is preferably from 200 to 400, and the
content of the thickener is an amount necessary to obtain this
consistency. The content is preferably from 3 to 30 mass %, more
preferably from 5 to 20 mass %, and most preferably from 7 to 15 mass %,
based on the entire grease composition.

[0035] [(B) Base Oil]

[0036] The base oil used in the grease composition for the ball bearing 1
of this illustrative embodiment is an ester synthetic oil obtained using
trimethylolpropane as an alcohol. The fatty acid constituting the ester
synthetic base oil includes a linear or branched fatty acid having a
carbon number of 6 to 20, preferably from 6 to 18. The fatty acid may be
a single fatty acid or a mixture of two or more fatty acids but is
preferably a mixture. A mixture of a linear fatty acid and a branched
fatty acid is more preferred, because as compared to a configuration
using a linear fatty acid alone, a mixture with a branched fatty acid
brings about excellent fluidity at a low temperature. The fatty acid may
be either saturated or unsaturated but is preferably a saturated fatty
acid.

[0037] Above all, an ester synthetic oil obtained from trimethylolpropane
and a mixture of a linear saturated fatty acid having a carbon number of
6 to 12 and a branched saturated fatty acid having a carbon number of 16
to 18 is preferred, and an ester oil of trimethylolpropane with a mixture
of n-octanoic acid, n-decanoic acid and i-stearic acid is most preferred.

[0038] The kinetic viscosity of the base oil at 40° C. is
preferably from 10 to 50 mm2/s, more preferably from 15 to 35
mm2/s.

[0039] [(C) Antioxidant]

[0040] The grease composition used for the ball bearing 1 of this
illustrative embodiment includes, as the essential component, (c-1) a
1-naphthylamine antioxidant and (c-2) a diphenylamine antioxidant.

[0041] The total amount of (c-1) and (c-2) ranges from 1.0 to 10 mass %,
preferably from 2.0 to 6.0 mass %, based on the entire mass of the grease
composition. When the total amount of (c-1) and (c-2) is in this range,
the rolling bearing having enclosed therein the grease composition is
excellent in the heat-stable acoustic life performance.

[0042] The blending ratio of (c-1) the 1-naphthylamine antioxidant and
(c-2) the diphenylamine antioxidant ranges from 30:70 to 70:30 in terms
of mass ratio. When the blending ratio of (c-1) and (c-2) is in this
range, an excellent heat-stable acoustic life performance is obtained
with a small added amount.

[0043] As (c-1) the 1-naphthylamine antioxidant, a compound represented by
the following formula (I) is preferred:

##STR00003##

[0044] In this formula (I), each of R1 and R2 are mutually independent and
may be a hydrogen atom, or a linear or branched alkyl group having a
carbon number of 1 to 12.

[0046] R1 is preferably a hydrogen atom, and R2 is preferably a branched
alkyl group having a carbon number of 1 to 12, more preferably a branched
alkyl group having a carbon number of 3 to 10, still more preferably a
1,1,3,3-tetramethylbutyl group. It is yet still more preferred that R1 is
a hydrogen atom and R2 is a 1,1,3,3-tetramethylbutyl group.

[0047] The bonding position of R1 is preferably the 4-position of the
phenyl group. The bonding position of R2 is preferably the 6-position of
the naphthyl group.

[0048] The compound of formula (I) is preferably
N-phenyl-1,1,3,3-tetramethylbutylnaphthalene-1-amine.

[0049] In the grease composition of this illustrative embodiment, as the
1-naphthylamine antioxidant, one compound may be used alone, or two or
more compounds may be used in appropriate combination.

[0050] As (c-2) the diphenylamine antioxidant, a compound represented by
the following formula (II) is preferred:

##STR00004##

[0051] In the formula (II), each of R3 and R4 are mutually independent and
may be a hydrogen atom, or a linear or branched alkyl group having a
carbon number of 1 to 12.

[0053] The alkyl group represented by each of R3 and R4 can be bonded at
an arbitrary position of the phenyl group but is preferably bonded at the
p-position with respect to the amino group. More preferably, the compound
is p,p'-dioctyldiphenylamine where each of R3 and R4 is an i-octyl group.

[0054] In the grease composition used for the ball bearing 1 of this
illustrative embodiment, as the diphenylamine antioxidant, one compound
may be used alone, or two or more compounds may be used in appropriate
combination.

[0055] The grease composition for use in the ball bearing 1 of this
illustrative embodiment may include, if appropriate, additives usually
used for a grease composition, in addition to the above-described
essential components. Examples of the additive include a rust inhibitor,
a metal deactivator, a cleaning dispersant, an extreme-pressure additive,
a defoaming agent, an anti-emulsifier, an oil improver, a wear inhibitor
and a solid lubricant. The amount added of such an additive is usually
from 0.01 to 10 mass %.

[0056] In particular, the ball bearing 1 of this illustrative embodiment
is preferably used as a rolling bearing in a small motor (for example, a
brushless motor or a fan motor) for a home appliance or an information
device.

[0057] The present invention is not limited to the illustrative embodiment
and specific working examples shown in the description of the present
invention, and various changes and modifications can be made therein
within the range of technical concept disclosed in the scope of claim for
patent.

[0058] For example, in the illustrative embodiment above, a ball bearing
is used as the rolling bearing, but the present invention is not limited
thereto and can be applied to other rolling bearings or roller bearings.

EXAMPLES

Test Grease Used in Ball Bearing of Example

Examples 1 to 5 and Comparative Examples 1 to 7

[0059] Lithium 12-hydroxystearate was mixed and dissolved under heating in
the base oil shown in the Table 1 and then cooled to make a base grease.
The antioxidant in a predetermined amount was mixed with the base oil,
and this mixture was added to the base grease and sufficiently mixed. The
resulting mixture was kneaded by a three-roll mill to produce a grease
composition having a mixing consistency of 250 (JIS K2220).

Comparative Example 8

[0060] In the base oil, cyclohexylamine and stearyl amine were reacted
with diphenylmethane diisocyanate to prepare a base grease. The
antioxidant in a predetermined amount was mixed with the base oil, and
this mixture was added to the base grease and sufficiently mixed. The
resulting mixture was kneaded by a three-roll mill to produce a grease
composition having a mixing consistency of 250 (JIS K2220).

[0062] The test grease was enclosed in a ball bearing (inner diameter: 8
mm, outer diameter: 22 mm, width: 7 mm) with a steel shield in an amount
of 25 to 35% of the bearing volume. This ball bearing was set in a
housing by two and after applying a preload of about 40 N, a shaft was
inserted into the inner diameter of the bearing. The shaft was connected
to the rotary shaft of a test motor to rotate the inner ring of the ball
bearing.

[0063] Subsequently, the ball bearing was put in a constant-temperature
chamber, and an evaluation test was performed at a test temperature of
110° C. and a rotation speed of 3,000 rpm by performing rotation
for 1,500 hours. Ten ball bearings were tested for each of Examples and
Comparative Examples, and the average value was determined.

[0064] The acoustic performance before and after rotation of the bearing
was evaluated by measuring the Anderon value of M band (300 to 1,800 Hz)
with use of an Anderon meter. The initial acoustic performance was
evaluated by the Anderon value before rotation of the ball bearing, and
the heat-stable acoustic performance was evaluated by the Anderon value
after rotation of the ball bearing for 1.500 hours. It is noted that the
M band frequency of 300 to 1,800 Hz is considered a sound offensive to
the human ear.

2. Evaluation Criteria of Initial Acoustic Performance

[0065] A: The average Anderon value was less than 0.3.

[0066] B: The average Anderon value was 0.3 or more.

3. Evaluation Criteria of Heat-Stable Acoustic Performance

[0067] A: The average Anderon value was less than 5.

[0068] B: The average Anderon value was 5 or more.

[0069] The results are shown in Table 1. In the Table, "mass %" is the
value based on the total mass of the grease composition.

[0070] In all of Examples 1 to 5 where a grease composition obtained by
using a lithium soap thickener, an ester synthetic base oil obtained by
an esterification reaction of trimethylolpropane with a fatty acid (Ester
Oil A), 1-naphthylamine (Antioxidant A) and diphenylamine (Antioxidant B)
each in a predetermined amount and combining these in a predetermined
ratio was enclosed, the initial acoustic performance was good, and the
heat-stable acoustic life performance was also good.

[0071] In Comparative Example 1 where a grease composition obtained by
blending the same amount of Antioxidant C in place of Antioxidant B in
Example 3 was enclosed, the heat-stable acoustic life performance was
poor.

[0072] In Comparative Example 2 (blending of Ester Oil B) where a grease
composition obtained by blending a base oil different from that in
Example 1 was enclosed, the heat-stable acoustic life performance was
poor.

[0073] In Comparative Example 3 (blending of Antioxidant A and Antioxidant
B in a total amount of 0.6 mass %) where a grease composition obtained by
blending Antioxidants A and B in an amount smaller than that specified in
the present invention was enclosed, the heat-stable acoustic life
performance was poor.

[0074] In Comparative Example 4 (blending of Antioxidant A and Antioxidant
B in a total amount of 14 mass %) where a grease composition obtained by
blending Antioxidants A and B in an amount larger than that specified in
the present invention was enclosed, the initial acoustic performance was
poor.

[0075] In both of Comparative Example 5 (blending of 0.4 mass % of
Antioxidant A and 1.6 mass % of Antioxidant B) and Comparative Example 6
(blending of 1.6 mass % of Antioxidant A and 0.4 mass % of Antioxidant
B), where a grease composition obtained by blending Antioxidants A and B
in the same total amount as in Example 1 but in a ratio out of the range
specified in the present invention was enclosed, the heat-stable acoustic
life performance was poor.

[0076] In Comparative Example 7 (blending of Antioxidant A) where a grease
composition obtained by blending the antioxidant in the same amount as in
Example 4 but not blending Antioxidant B was enclosed, the heat-stable
acoustic life performance was poor.

[0077] In Comparative Example 8 (urea) where a grease composition obtained
by blending a thickener different from that in Example 4 was enclosed,
the heat-stable acoustic life performance was poor.

[0078] These results reveal that the rolling bearing having enclosed
therein the grease composition of the present invention has both
excellent initial acoustic performance and excellent heat-stable acoustic
life performance.